Neutron Inelastic Scattering Studies of Globular Compounds

The use of neutron inelastic scattering measurements to determine the nature of the solid-state transitions in globular compounds is proposed and exemplified. Patterns were taken of the high- and low-temperature phases of solid cyclohexane, 2–2-dimethylbutane, and 1–4-diazo-bicyclo-(2.2.2)-octane. The first two have broad inelastic scattering peaks in the low-temperature phase. In the high-temperature phase these peaks disappear and the elastic peak broadens markedly. These changes are attributed to ``quasi-free-rotation'' of the molecules. Inelastic peaks do not appear in 1–4-diazo-bicyclo-(2.2.2)-octane, and the elastic peak does not broaden appreciably in the high-temperature phase. Self-diffusion coefficients Ds=4.5×10−5 cm2/sec for cyclohexane and Ds=8.6×10−5 cm2/sec for 2–2-dimethylbutane were determined from the patterns of the liquids at 300°K using the ``small motions'' approximation. These values are larger than those obtained from NMR measurements, presumably as a result of irreversible rotation of the molecules.